Optical waveguides can be used to expand or replicate the exit pupil of an imaging system in one or two dimensions. Typically, light from the exit pupil of the imaging system is received in the waveguide through an entrance or in-coupling, and travels through the waveguide in a direction, while some of the light exits a grating structure of the waveguide to a display or the eye of a user. The remaining light that does not exit the grating structure may reflect off of the internal surfaces of the waveguide before finally exiting through the grating structure or some other part of the waveguide.
One issue with current waveguide-based exit pupil expanders is they may result in visible interference when used with coherent light sources. Coherent light sources are light sources containing a narrow range of frequencies. Coherent light sources when split into multiple paths by the waveguide will generally interfere with themselves when received by the display if the difference in optical path length traversed by each optical path of the light is less than a coherence length associated with the coherent light source. An example of a coherent light source is a laser.
Coherent light sources have several advantages over current LCoS (liquid crystal on silicon) based imaging systems, especially when used in head mounted display systems. For example, they have high sequential contrast ratios, lower weight and size, and can be adjusted to compensate for non-uniformities of the waveguide. Accordingly, there is a need to reduce the visible interference associated with coherent light sources and optical waveguides.